1. Field of the Invention
This invention relates to improvements in voltage regulator circuits and more particularly to improvements in voltage regulator circuits of the type that maintain the system voltage in a vehicle, or the like, in which electrical loads may be applied to a vehicle alternator at various engine speeds.
2. Description of the Prior Art
Electrical systems of today's vehicles typically use an alternator to convert the mechanical energy transmitted into it via its rotating shaft into electrical energy. The amount of current delivered by an alternator to its load is determined by many factors, such as the rotational speed of the shaft, the voltage of the system, and the amount of current flowing in the field coil of the alternator.
On the other hand, the amount of mechanical energy taken in by the alternator is nearly a linear function of the electrical energy delivered by it to the load: if the load draws twice as much electrical power, the alternator will pull approximately twice as much mechanical power in through its shaft. For a given shaft speed, the different levels of mechanical power are exhibited by variations in torque seen at the mechanical shaft of the alternator.
The rotating part of an alternator spins a coil which is energized via slip rings and brushes. This rotating coil, the "rotor" or "field" coil, sets up a magnetic field which moves through stationary coils, the "stator" coils, whose output is then delivered to the load via rectifiers. The output of the alternator is controlled by a voltage regulator which senses the system voltage and varies the field coil current to set the system voltage to some predetermined value. If the electrical load presented by the vehicle increases, the voltage regulator increases the field coil current, which increases the output current from the stator coils of the alternator until the system voltage is restored to its proper value. When the field coil current is increased, the torque exerted by the alternator through its input shaft and pulley increases.
Rapid changes in the amount of energy pulled by the alternator from the engine through its belt drive can cause several problems. Two that may be noticeable by the operator of the vehicle are belt squeal when the torque is rapidly increased to very high levels and idle surge. The idle surge problem is caused by the fact that modern day engines are made to idle at very low speeds, generally under computer control, to minimize polluting emissions. An engine that is idling may be putting out a total of 2 or 3 horsepower in mechanical energy used to overcome friction within the engine itself and the transmission, drive the alternator, and operate other rotating components on the engine, such as the water pump, air pump, air conditioning compressor, and so forth. The alternator may draw anywhere from 0.1 to 2 horsepower, depending on the electrical load requirements at the time. A rapid increase in the electrical loads, such as a electrically driven cooling fan or lights turning on may cause the alternator to rob the engine of up to half its output. This could push the engine close to stalling, causing the engine control computer to kick up the throttle to prevent the stall. Often the system overshoots and the idle speed briefly surges up to a higher than normal level. This is an undesirable condition.
Due to power dissipation problems, most existing alternator voltage regulators drive the field coil with a pulse width modulated square wave. The frequency of the drive is high enough that the field coil current changes very little during one cycle of the drive. Varying the duty cycle (the ratio of on to off time) changes the average level of field coil current. However, controlling the rate of change of the field coil current will control the rate of change in the torque seen by the engine.
In the past, several systems have been proposed to address these problems. One system proposed by Kirk, et al. in U.S. Pat. No. 4,459,489 uses a constantly fixed rate of increase for the field coil drive. The Kirk, et al. circuit controls the rate of increase in the field drive to a constant maximum at all times. The reaction speed of the system is not determined by the size of the electrical load increase, or by the shaft speed of the alternator. Such systems could suffer from voltage stability problems, because they react slowly to load increases, causing the system voltage to dip to an unnecessary extent, even in response to only small load increases. Additionally, in many such systems, the reaction speed is constant for all engine speeds. When the engine is running faster, it develops a lot of horsepower and is relatively unaffected by the load changes from the alternator. Additionally, the higher rotational velocity means that the belt drive to the alternator experiences less available to transmit a given amount of mechanical energy to the alternator.
Another system that has been proposed by Bowman et al. in U.S. Pat. No. 4,636,706 uses predetermined update rates for the field coil drive. The Bowman et al. circuitry has a reaction time dependent on the shaft speed of the alternator with a piece-wise linear relationship. In this system, if the shaft speed is within a certain range, the rate of increase of the field drive duty cycle has a certain value. For a different shaft speed, the field drive increase will have a different value. At the transition between the two ranges, the field drive increase rate has a step change in value. Implementing this approach generally requires frequency discriminators for sensing the shaft speed. If the alternator is operated at one of the transitions, erratic operation can result.